1. Field of the Invention
The present invention relates generally to the field of instrumented test hammers, and, more particularly, to an improved test hammer which avoids spurious electrical signals and glitches due to post-impact vibration of the head assembly.
2. Description of the Prior Art
Instrumented test hammers for exciting a test object have been developed heretofore. Such prior art test hammers have included a head assembly, a handle extending away therefrom, and a graspable grip mounted on the distal end of the handle. The head assembly included an impact tip, and one or more piezo-electric crystals held under a preload between the tip and another mass. Thus, when the hammer was caused to impact against an object, the piezo-electric crystals would produce an analogous electrical signal proportional to the force of the impact. Prior art grips were of a relatively-hard resilient material having a hardness of about 78 on a Shore "A" scale when measured with a durometer.
Details of such earlier hammers, and their uses and applications, are representatively shown and described in one or more of the following prior art references: Halverson and Brown "Impulse technique for structural frequency result testing", Sound and Vibration, November 1977 (pp 8-21); Lally, "Testing the Behavior of Structures", Test, August/September 1978; R. W. Lally, "Transduction", PCB Piezotronics, Inc. (1981); "Stress Waves in a Long Bar", PCB Piezotronics, Inc. (1974); "Piezo-electric Analogies", Electromechanical Design, December 1967 (pp. 52-53).
However, upon information and belief, such prior art instrumented test hammers were somewhat limited in their application and use because the data produced thereby depended largely upon the expertise of the particular operator, which varied from individual to individual. The operator's hand and arm often became, in effect, part of the hammer structure, and introduced additional mass, stiffness and damping. At times, this enhanced post-impact vibration of the head assembly, and frequently caused spurious oscillation of the generated electrical signal, which destroyed the proportionality of the signal to the force of impact. Sometimes, the dynamic behavior of such hammers caused an undesirable "double hit" against the object, with the production of a corresponding oscillation in the spectrum of the electrical signal. Also, resonances of the hammer structure appearing in the test results were often mistaken for dynamic behavior of the object being tested.